1. Field of the Invention
The present invention relates to a switchboard control system, and more particularly to a switchboard control system for a time division exchange in which a plurality of switchboards are connected to a time division network.
2. Description of the Prior Art
Generally, in a toll switch and in an international switch, equipment associated with a switchboard is set up as an annex to a time division exchange for calls that are required to be connected by the switchboard operator.
In this kind of switchboard control system, as described, for instance, in "NXR-20 International Telephone Switching System", NEC RESEARCH & DEVELOPMENT, No. 54, July, 1979, p88-105, each switchboard has a voice signal line connected to a network and a control signal line connected to a central controller, and the connection between a trunk, the switchboard and a position trunk connected to in a time or space division network is carried out by controlling the network with the central controller.
This will be described in more detail. When the central controller of a time division exchange, decides that an incoming call from a trunk line needs connection by a switchboard, the central controller sends out incoming indication information to one of a plurality of switchboards that is selected following a predetermined procedure. As soon as the operator of the switchboard answers using an answer key, the central controller detects the operation of the answer key, acquires an idle position trunk (having three terminals F, R and P) and controls the network to connect the terminal F of the position trunk and the trunk connected to the incoming trunk line and to connect the terminal P of the position trunk and the switchboard that the central controller. The operator of the switchboard asks the calling party the classification of the call, the calling party's number, the called party's number and so forth and then operates an input key to enter information necessary for preparing a toll ticket. The central controller, after detecting information input on the operation of the input key, sends the toll ticket information to a display on the switchboard for display. At the same time, the information is stored in a memory. Upon the actuation of a called party select key by the operator of the switchboard, the central controller detects the operation of the called party select key, acquires a trunk connected to an idle trunk line connected to the called subscriber, controls the network to connect the acquired trunk and the terminal R of the position trunk, and performs processing for connection of the called party. The central controller detects an answer from the called party via the trunk and controls a predetermined lamp on the switchboard to indicate the detection. When the operator operates a disconnect key after confirming communication between the calling and the called party, the central controller detects the operation of the disconnect key and controls the network, disconnecting the terminal P of the position trunk from the switchboard.
As will be appreciated from the above description, the prior art switchboard control system has the following disadvantages:
First, each switchboard must be provided with a voice signal line connected to the network and, consequently, the voice signal line and a trunk are needed between each switchboard and the network, making the system configuration uneconomical.
Secondly, a control signal line must be provided between each switchboard and the central controller for transmitting operation information for various keys of the switchboard, display information for various display instruments and information for exchange processing. Since the information to be transmitted includes a large quantity of information such as toll ticket information or directory assistance information, large-capacity information transmitting means is required; in addition, the transmission admits of no delay. If a known common bus transmission system is employed for the control signal line, then its length is limited, imposing undesirable limitations on the distance between the switchboard and the time division exchange.
Third, if a MODEM is introduced into the control signal lin to increase the length between the switchboard and the automatic exchange, then the transmission rate is reduced and, an increase in the number of such MODEMs used to compensate for the reduced transmission rate would be more uneconomical.